This invention relates to DC link variable speed constant frequency (VSCF) power systems having at least two parallel connected channels, and more particularly, to a method for reducing unbalanced current flow in such systems and a circuit which performs that method.
AC electric power systems are usually connected in parallel to increase total system rating or, in certain cases such as airborne power systems, to increase reliability. In order to further improve reliability and to maximize efficiency, it is generally desired that the total system load be divided equally among the paralleled channels. In any paralleled system, AC or DC, the load sharing among channels can be accomplished by inserting impedance in series with each channel. Such impedance is usually objectionable because in addition to forcing current sharing, it creates unwanted voltage droop at the point of regulation where the channels are tied together. By sensing difference currents between participating channels and applying this error current in a manner which minimizes the system unbalance, it is possible to effectively insert a "paralleling" impedance which appears only between the channels but not in the load circuit. This "paralleling" impedance is an apparent impedance rather than a real lumped impedance and is simulated in the controls rather than in the power circuit of the power supply
In paralleled DC systems, only the magnitude of the voltage must be controlled to minimize unbalanced current error since the DC voltage source is defined by its magnitude. In an AC paralleled system, two parameters, magnitude and phase angle, must be controlled. It is assumed that in AC systems, all of the paralleled channels are operating at the same frequency.
The exact method of control of AC systems is dependent upon the impedances between the paralleled source Thevenin voltages. For example, if the impedance between sources is primarily resistive, then a difference in Thevenin voltage magnitude will create an unbalance in real power and a phase angle unbalance will create an unbalance in reactive power. The impedance between sources includes the source or Thevenin impedance, feeder bus impedance, and any parallel tie bus impedances.
DC link VSCF systems have output filters consisting of a series inductor and a shunt capacitor. The output impedance of a DC link VSCF system therefore looks capacitive or leading as viewed from the load. The difference impedance for paralleling purposes, however, does not include the filter capacitor or other shunt connected load impedances. Only series connected impedances are important when viewed from source to source. The actual difference impedance for any one channel includes both its filter inductor impedance and the feeder impedance to the point of load connection. This is very similar to constant speed generator type systems which by their nature are inductive machines. In fact, the magnitude of the difference impedance is similar because a VSCF filter inductor is approximately 0.1 per unit, a value which is typical for a constant speed generator subtransient reactance.
Because of this inductive impedance, small phase angle errors result in difference currents which are in phase with the Thevenin voltage of the power sources and magnitude errors result in error current which is in quadrature with the Thevenin voltage. Hence the DC link VSCF parallel control system must use phase angle for real load division control and magnitude for reactive load control. This is identical to the conventional constant speed generator type systems which have been in use for many years.
When parallel connected DC link VSCF power sources are operated at no load or are very lightly loaded, a problem results from the fact that the DC link VSCF power sources have a discontinuous real power characteristic. In fact, the VSCF inverters cannot pass negative power from the AC output side to the generator shaft input side, due to the presence of a DC link rectifier. The reactive power characteristics, on the other hand, are linear through zero and are unaffected by the DC link rectifier. A conventional constant speed synchronous generator parallel system has continuous linear functions through zero for both real and reactive power. This inherent difference between the two systems means that the DC link VSCF parallel link controls cannot be the same as the parallel link controls used on a conventional constant speed drive parallel system.